The securing function of intramedullary nails is already known in the state of the art. The locking screws or locking pins (hereinafter only the term “locking screw” is used, but this is also intended to include the term other fasteners, such as locking pins) are inserted into the transverse bores in an intramedullary nail either with the help of an imaging method (x-ray monitoring) or a fairly complex targeting device. In both cases, a certain target inaccuracy is unavoidable, i.e., the tip of the screw cannot be aligned precisely coaxially with the central axis of the transverse bore but instead there is a certain deviation. In order for the locking screw to open into and pass through the transverse bore despite this target error, the outside diameter of the screw is undersized in relation to the diameter of the transverse bore. If the target inaccuracy remains within the scope of this undersizing, then the locking screw can be passed through the transverse bore with no problem despite this target error. However—because of the undersizing—the locking screw does have a certain amount of play in relation to the transverse bore.
This play defines the allowed amount of movement of the main bone fragments, which are to be secured in the corresponding locking hole by means of locking screws, in relation to the nail, and therefore, owing to the rigidity of the nail, also in relation to other main bone fragments attached with the same nail. This play is unavoidable in order to be sure that the surgeon will be able to use this locking means, but is nevertheless undesirable clinically for certain indications (e.g., in the case of metaphysical fragments).
Nails having a solid cross section, which may have an inside thread in the locking hole, are also not free of play. The inside thread only prevents the nail from being displaced axially on the locking screw.
US 2002/173792 (Hover et al.) describes a hollow intramedullary nail made of metal having one or two plastic inserts in the jacket openings in the transverse bore, which are positioned diametrically opposite one another and are referred to as windows, the locking screw being insertable through these plastic inserts. This known intramedullary nail has the disadvantage that the window-like plastic inserts are easily pushed in, causing the desired function to be lost. But even with very careful manipulation, the two plastic inserts could be forced out of their “windows” when the locking screw is passed through, which also causes a loss of function.
In a special embodiment of the Hover patent, an insert on the distal tip of the intramedullary nail that can be inserted into the central cavity thereof is provided in both windows in the transverse opening, but its material composition and function remain unclear.